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For the full breakdown, see our best Battery Backup For Home Appliances guide.
The question of how to size a battery backup for home appliances usually comes down to two numbers: total load in volt-amps (VA) and the runtime you need when the power goes out. A battery backup for this purpose is an uninterruptible power supply (UPS), and both numbers are measurable before you buy. Get them right and the arithmetic is simple; get them wrong and the unit either trips under load or costs more than it needed to.
That calculation covers connected equipment capacity, but it does not set runtime. A large battery backup on a heavy load can run out quickly; the same unit on a lighter load lasts far longer. So the practical order is: list the equipment, add the growth margin, then choose the battery size that supports that load for the outage duration you care about.
Sizing a Home Battery Backup Starts With the Load List
Start by listing every appliance that will plug into the backup, then read each nameplate and multiply volts by amps. That product is the VA load.
Read the nameplate on the back